Magnetic field measurement apparatus

ABSTRACT

A magnetic field measurement apparatus includes an irradiation portion, a gas cell, a measurement unit (polarization separation unit, light receiving portion, signal processing circuit), and a magnetic shield. The magnetic shield is formed in a elongated hollow shape having openings at both sides thereof. The gas cell, in which gaseous atoms are sealed, is disposed in a hollow area of the magnetic shield. The irradiation portion irradiates irradiation light including linearly polarized light adjusted so that the vibration direction of an electric field coincides with the axis direction of the magnetic shield onto the gaseous atoms sealed in the gas cell along a direction perpendicular to the axis of the magnetic shield. The measurement unit measures a rotational angle of a polarization plane of the irradiation light that has been irradiated by the irradiation portion and passed through the gaseous atoms.

BACKGROUND

1. Technical Field

The present invention relates to magnetic field measurement apparatusesmaking use of light.

2. Related Art

Magnetic field measurement apparatuses making use of light areapparatuses that measure minute magnetic fields generated frombiological objects, such as a magnetic field from the heart(magneto-cardiogram) and a magnetic field from the brain(magneto-encephalogram), and are expected to be applied in medical imagediagnostic apparatuses and the like. An element in which a gas of alkalimetal atom or the like is sealed is used in measuring a magnetic field.By irradiating pump light on the element mentioned above, energy of theatom in the element is excited in accordance with the magnetic field anda polarization plane of probe light having passed through the element isrotated due to a magneto-optic effect. A magnetic field measurementapparatus measures a rotational angle of the polarization plane asmagnetic field information.

Researches to enhance the sensitivity of the magnetic field measurementapparatus have been carried out. A light pumping magnetometer that makesit possible to detect a magnetic field at a higher level of sensitivityby increasing the intensity of a detection signal is disclosed inJP-A-2009-236599.

However, the magnetometer disclosed in JPA-2009-236599 has a problem inthat its sensitivity is lowered if there exists a magnetic fielddisturbance in a direction other than the direction of a magnetic fieldto be measured.

SUMMARY

An advantage of some aspects of the invention is to measure a magneticfield while suppressing the influence of a magnetic field disturbancethat exits in a direction other than the direction of the magnetic fieldto be measured.

In order to solve the above problem, a magnetic field measurementapparatus according to an aspect of the invention includes: a magneticshield that is formed in a elongated hollow shape having openings atboth sides thereof; a gas cell in which gaseous atoms are sealed andthat is disposed in a hollow area of the magnetic shield; an irradiationunit that irradiates irradiation light including linearly polarizedlight adjusted so that the vibration direction of an electric fieldcoincides with the axis direction of the magnetic shield onto thegaseous atoms sealed in the gas cell along a direction perpendicular tothe axis of the magnetic shield; and a measurement unit that measures arotational angle of a polarization plane of the irradiation light whichhas been irradiated by the irradiation unit and passed through thegaseous atoms.

With the configuration described above, a magnetic field can be measuredwhile suppressing the influence of a magnetic field disturbance that ispresent in a direction other than the direction of the magnetic field tobe measured.

It is preferable for the irradiation unit to guide the irradiation lightto the gas cell with an optical fiber.

With the above configuration, limitations on size, arrangement and so onof the irradiation unit can be reduced in comparison with a case inwhich the irradiation light is guided to the gas cell without using anoptical fiber.

A magnetic field measurement apparatus according to another aspect ofthe invention includes: a magnetic shield that is formed in a elongatedhollow shape having openings at both sides thereof; a gas cell in whichgaseous atoms are sealed and that is disposed in a hollow area of themagnetic shield; a pump light irradiation unit that irradiates pumplight including a circularly polarized light component onto the gaseousatoms sealed in the gas cell along a direction parallel to the axis ofthe magnetic shield; a probe light irradiation unit that irradiatesprobe light including linearly polarized light onto the gaseous atomssealed in the gas cell along a direction perpendicular to the axis ofthe magnetic shield; and a measurement unit that measures a rotationalangle of a polarization plane of the probe light which has beenirradiated by the probe light irradiation unit and passed through thegaseous atoms.

With the configuration described above, a magnetic field can be measuredwhile suppressing the influence of a magnetic field disturbance that ispresent in a direction other than the direction of the magnetic field tobe measured.

It is preferable for the pump light irradiation unit to guide the pumplight to the gas cell with an optical fiber and for the probe lightirradiation unit to guide the probe light to the gas cell with anoptical fiber.

With the above configuration, limitations on size, arrangement and so onof the pump light irradiation unit and the probe light irradiation unitcan be reduced in comparison with a case in which the pump light and theprobe light are not guided to the gas cell with optical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a diagram illustrating an overall configuration of a magneticfield measurement apparatus according to a first embodiment of theinvention.

FIG. 2 is a diagram illustrating an exterior appearance of a magneticshield.

FIGS. 3A and 3B are diagrams for explaining a principle of magneticfield measurement by a one-beam type measurement apparatus.

FIGS. 4A through 4C are diagrams for explaining magnetic fieldmeasurement by a measurement apparatus without having the features thatare included in the magnetic field measurement apparatus according tothe first embodiment.

FIGS. 5A through 5C are diagrams for explaining magnetic fieldmeasurement by the magnetic field measurement apparatus according to thefirst embodiment.

FIG. 6 is a diagram illustrating an overall configuration of a magneticfield measurement apparatus according to a second embodiment of theinvention.

FIG. 7 is a diagram for explaining a principle of magnetic fieldmeasurement by a two-beam type measurement apparatus.

FIGS. 8A and 8B are diagrams for explaining a trajectory of amagnetization vector according to the second embodiment.

FIGS. 9A and 9B are diagrams for explaining magnetic field measurementby a measurement apparatus without having the features that are includedin the magnetic field measurement apparatus according to the secondembodiment.

FIGS. 10A through 10C are diagrams for explaining magnetic fieldmeasurement by the magnetic field measurement apparatus according to thesecond embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS 1. First Embodiment 1-1.Configuration

FIG. 1 is a diagram illustrating an overall configuration of a magneticfield measurement apparatus 9 according to a first embodiment of theinvention. The magnetic field measurement apparatus 9 is what is knownas a one-beam type measurement apparatus in which irradiated probe lightserves as both probe light itself and pump light. An irradiation portion1 includes a light source 11 and a converter 12. The light source 11 isa laser generation device that generates a laser of a frequency thatcorresponds to a hyperfine structure level transition of the atom sealedin a gas cell 2 explained later. The converter 12 includes apolarization plate and the like and converts the laser generated by thelight source 11 to irradiation light including a linearly polarizedlight component in a predetermine direction. The irradiation lighthaving been generated by the light source 11 and converted by theconverter 12 is guided by an optical fiber or the like, for example, andirradiated to the gas cell 2. The irradiation light may be irradiateddirectly from an irradiation unit or the like without using an opticalfiber or the like by disposing the irradiation portion 1 inside amagnetic shield 7 explained later, for example. However, limitations onsize, arrangement and so on of the irradiation portion 1 are reduced byguiding the irradiation light using an optical fiber or the like.

The gas cell 2 is a cell (element) made of glass in which atoms ofalkali metal such as potassium (K), cesium (Cs) or the like are sealed.The gas cell 2 transmits the above described irradiation lightirradiated from the irradiation portion 1 therethrough. The irradiationlight having passed through the gas cell 2 is guided to a polarizationseparation unit 3 with an optical fiber or the like. Here, the materialof the gas cell 2 is not limited to glass, and a material such as aresin or the like may be employed as long as the material transmitsirradiation light. Note that the irradiation light having passed throughthe gas cell 2 may be irradiated directly from an irradiation unit orthe like without using an optical fiber.

The polarization separation unit 3 separates the irradiation lighthaving passed through the gas cell 2 in accordance with a polarizationdirection. Specifically, the polarization separation unit 3 transmits alinearly polarized light component arranged in the same direction as thelinearly polarized light component included in the irradiation lightbefore being converted by the converter 12, and reflects light having apolarized light component in a direction perpendicular to thepolarization direction of the above linearly polarized light component.

A light receiving portion 4 includes a transmission light receivingelement 41 and a reflection light receiving element 42. The transmissionlight having passed through the polarization separation unit 3 isreceived by the transmission light receiving element 41, while thereflection light having reflected off the polarization separation unit 3is received by the reflection light receiving element 42. Thetransmission light receiving element 41 and the reflection lightreceiving element 42 respectively generate a signal in accordance withthe received light quantity and supply the generated signal to a signalprocessing circuit 5. The signal processing circuit 5 receives the abovesignal from each of the transmission light receiving element 41 and thereflection light receiving element 42. Then, based on the received eachsignal, the signal processing circuit 5 measures an amount of rotationof the linearly polarized light component included in the irradiationlight that has been irradiated by the irradiation portion 1, i.e.,measures a rotational angle of a polarization plane thereof; therotation is made while the linearly polarized light component passingthrough the gas cell 2. The polarization separation unit 3, the lightreceiving portion 4, and the signal processing circuit 5 function as ameasurement unit that measures a rotational angle of the polarizationplane of the irradiation light having been irradiated by the irradiationportion 1 and passed through the gaseous atoms. A display unit 6displays the rotational angle of the polarization plane measured by thesignal processing circuit 5.

The magnetic shield 7 indicated in a dot-dot-dash line in FIG. 1 is ashield formed in a elongated hollow shape having openings at both endsthereof and suppresses a magnetic field of an exterior area frompenetrating into a hollow area of the shield. It is to be noted that theposition of the magnetic shield 7 illustrated in FIG. 1 does notrepresent the actual position of arrangement with respect to theirradiation portion 1, the gas cell 2 and the polarization separationunit 3.

The material of the magnetic shield 7 contains magnetized materials madeof permalloy, amorphous matter of iron, chromium, cobalt or the like,sintered ferrite and so on. FIG. 2 is a diagram illustrating an exteriorappearance of the magnetic shield 7. Hereinafter, in order to describethe arrangement of constituent elements of the magnetic fieldmeasurement apparatus 9, the space in which these constituent elementsare arranged is expressed as an xyz coordinate space of right-hand inthe drawings. Among the coordinate symbols indicated in the drawings, asymbol formed of a white circle with a black circle inside thereofrepresents an arrow coming from the depth side to the front side of thepaper; a symbol formed of a white circle with two lines intersectingeach other inside thereof represents an arrow going from the front sideto the depth side of the paper. In the coordinate space, a directionalong an x-axis is called an x-axis direction. In addition, of thex-axis direction, a direction along which an x component increases iscalled a +x direction, whereas a direction along which an x componentdecreases is called a −x direction. Likewise, as for y and z components,a y-axis direction, a +y direction, a −y direction, a z-axis direction,a +z direction, and a −z direction are defined respectively.

Note that, in the following explanation, when an expression such that acertain direction is arranged along a certain axis is described, theexpression includes a case where “the certain direction” is exactlyparallel to “the certain axis”; however, the expression is not limitedto this case, and includes a case such that a certain direction and acertain axis can be considered to be parallel to each other as long asthe same effect can be obtained as in the case where both are exactlyparallel to each other. The case in which a certain direction and acertain axis can be considered to be parallel to each other is, forexample, a case where an angle formed by the two is within a range of ±2degrees.

As shown in FIG. 2, the axis connecting the openings at both ends of themagnetic shield 7 is arranged along the y-axis. The gas cell 2 isdisposed in the hollow area of the magnetic shield 7.

The magnetic field measurement apparatus 9 has a feature in which thevibration direction of an electric field of linearly polarized lightincluded in the irradiation light which has been converted by theconverter 12 is so configured as to coincide with the axis direction ofthe magnetic shield 7.

1-2. Operation

FIGS. 3A and 3B are diagrams for explaining a principle of a magneticfield measurement by what is known as a one-beam type measurementapparatus. In this one-beam type measurement apparatus, when linearlypolarized light is irradiated onto gaseous atoms sealed in the gas cell,the gaseous atoms are optically pumped and the probability distributionof magnetic moment generated when the energy changes is caused to varyfrom a spherical origin-symmetry distribution. For example, in the caseof an energy transition of hyperfine structure level where F→F′=F−1, theprobability distribution of magnetic moment of the gaseous atoms takes aform corresponding to a region R₁ that stretches along a vibrationdirection of the linearly polarized light. This probability distributionis called “alignment”. That is to say, as shown in FIG. 3A, if linearlypolarized light whose electric field vibration direction is arrangedalong an arrow D₀ direction that is parallel to the +y direction isirradiated toward the +x direction, for example, alignment that isdistributed in the region R₁ along the y-axis direction is generated inthe gaseous atoms through which the linearly polarized light passes.When the linearly polarized light passes through the gaseous atoms, itspolarization plane is rotated due to linear dichroism. FIG. 3Billustrates a rotational angle a of the polarization plane when thevibration direction of an electric field of the linearly polarized lightis viewed while facing the −x direction. Because the rotational angle αcorrelates with the strength of a magnetic filed in the +x direction,the strength of the magnetic field in the x-axis direction inside thegas cell 2 is determined by measuring the rotational angle α. Note thatin a one-beam type measurement apparatus, a direction D_(M) of amagnetic field to be measured is a direction toward which light isirradiated (+x direction in FIG. 3).

Here, in order to explain the above-mentioned features of the magneticfield measurement apparatus 9 according to the invention, operations ofa measurement apparatus that does not have these features will bedescribed first. FIGS. 4A through 4C are diagrams for explainingmagnetic field measurement by a measurement apparatus without having thefeatures that are included in the magnetic field measurement apparatus9. This measurement apparatus is configured by the same constituents asthe magnetic field measurement apparatus 9, but the vibration directionof an electric field of linearly polarized light which is included inirradiation light is not so configured as to coincide with the axisdirection of the magnetic shield. For example, in this measurementapparatus, the axis of the magnetic shield is arranged along the y-axisdirection shown in FIGS. 4A through 4C, and irradiation light having alinearly polarized light component is irradiated toward the +x directionsince the direction D_(M) of the magnetic field to be measured is set inthe +x direction. However, the vibration direction of the electric fieldof the irradiated linearly polarized light does not coincide with theaxis direction of the magnetic shield (y-axis direction). Specifically,the vibration direction mentioned above is in parallel with the z-axisdirection. Accordingly, in the gaseous atoms sealed in the gas cell,alignment is generated in a region R_(a) indicated in a broken line inFIGS. 4A through 4C. This alignment takes a form of approximatelyrotational symmetry central to the z-axis, and stretches along thez-axis direction.

Since this measurement apparatus includes the magnetic shield with theaxis that is arranged along the y-axis direction, disturbance inmagnetic fields along the x-axis direction, the z-axis direction and aresultant direction of the x-axis and y-axis directions can besuppressed at a high probability. However, a magnetic field disturbancealong the y-axis direction is likely to exert an influence on the hollowarea of the magnetic shield. The alignment generated in the gaseousatoms sealed in the gas cell as described above rotates central to anaxis parallel to the y-axis upon receiving a magnetic field disturbancealong the y-axis direction. That is, the alignment rotates so as to havea posture corresponding to a region R_(b) indicated in a solid line inFIG. 4A. FIG. 4B shows the alignment that is viewed while facing the +ydirection. As described thus far, the alignment that has originally beengenerated in the region R_(a) is made to rotate in an arrow D₄ directionbeing influenced by the magnetic field in the y-axis direction that hasnot been suppressed sufficiently by the magnetic shield, andconsequently has a posture corresponding to the region R_(b).

FIG. 4C shows the alignment that is viewed from the side where theirradiation light having passed through the gaseous atoms is measured,i.e., the alignment is viewed from the +x direction side while facingthe −x direction side on which the gaseous atoms exist. The regionR_(b), which shows a form of the alignment after having been rotated dueto the magnetic field disturbance, is shorter in length when viewedfacing the −x direction than the region R_(a), which shows a form of thealignment before the rotation. In other words, the length of the regionRa in the z-axis direction is L₀, whereas the length of the region R_(b)in the z-axis direction is L₁, which is shorter than L₀. Accordingly, inthis measurement apparatus, the size of the alignment when viewed fromthe side where the irradiation light having passed through the gaseousatoms is measured becomes smaller being influenced by the magnetic fielddisturbance. As a result, the sensitivity of this measurement apparatusis lowered due to the influence of the magnetic field disturbance.

Meanwhile, FIGS. 5A through 5C are diagrams for explaining magneticfield measurement by the magnetic field measurement apparatus 9according to the invention. The axis of the magnetic shield 7 in themagnetic field measurement apparatus 9 is arranged along the y-axisdirection shown in FIGS. 5A through 5C, and irradiation light includinga linearly polarized light component is irradiated toward the +xdirection because the direction D_(M) of the magnetic field to bemeasured is set in the +x direction. The vibration direction of anelectric field of the irradiated linearly polarized light is made tocoincide with the axis direction of the magnetic shield (y-axisdirection). Accordingly, alignment is generated in the region R₁ asshown in FIG. 5A in the gaseous atoms in the gas cell. This alignmenttakes a form of approximately rotational symmetry central to the y-axisand stretches in the y-axis direction. Since the alignment makes Larmorrotation central to the x-axis, the rotational symmetry deviates fromthe y-axis. However, because its deviation displacement is much smallercompared to size of the magnetic field disturbance, this alignment canbe considered to have a form of approximately rotational symmetrycentral to the y-axis.

Since the magnetic field measurement apparatus 9 has the magnetic shield7 whose axis is arranged along the y-axis direction, a magnetic fielddisturbance along the y-axis direction is more likely to influence thehollow area than those along the directions of x-axis, z-axis and thelike. The aforementioned alignment generated in the gaseous atoms in thegas cell 2 rotates about the y-axis direction as an axis upon receivinga magnetic field disturbance along the y-axis direction. In other words,the alignment rotates in an arrow D₅ direction as shown in FIG. 5A. FIG.5B shows the alignment that is viewed while facing the +y direction. Asdescribed above, the alignment generated in the region R₁ takes a formof approximately rotational symmetry central to the y-axis and stretchesin the y-axis direction. Therefore, even if the alignment is rotated inthe arrow D₅ direction following a trajectory of an arc central to they-axis, its form hardly varies. Accordingly, as shown in FIG. 5C, thelength in form of the alignment that is viewed while facing the −xdirection hardly differs between before and after the rotation due tothe magnetic field disturbance. In other words, the length L₀ of thealignment before the rotation is approximately equal to the length L₁ ofthe alignment after the rotation. That is to say, sensitivity of themagnetic field measurement apparatus 9 is unlikely to be influenced by amagnetic field disturbance.

2. Second Embodiment 2-1. Configuration

FIG. 6 is a diagram illustrating an overall configuration of a magneticfield measurement apparatus 9A according to a second embodiment of theinvention. The magnetic field measurement apparatus 9A is what is knownas a two-beam type measurement apparatus in which a pump lightirradiation device and a probe light irradiation device are separatelyprovided. The magnetic field measurement apparatus 9A is different fromthe magnetic field measurement apparatus 9 of the first embodiment inthat the apparatus 9A has a probe light irradiation portion 1A in placeof the irradiation portion 1 and a pump light irradiation portion 8;other constituents of the magnetic field measurement apparatus 9A arethe same as those of the magnetic field measurement apparatus 9. It isto be noted that the magnetic shield 7 in FIG. 6 is not placed at theactual position with respect to the probe light irradiation portion 1A,the gas cell 2, the pump light irradiation portion 8 and thepolarization separation unit 3.

The description of the magnetic field measurement apparatus 9A is givenbelow focusing mainly on the features that are different from those inthe magnetic field measurement apparatus 9.

The probe light irradiation portion 1A includes a light source 11A and aconverter 12A. The light source 11A may be the light source 11, and theconverter 12A may be the converter 12. However, it is preferable that awavelength of probe light irradiated by the probe light irradiationportion 1A be set separated from the resonant frequency of gaseous atomssealed in the gas cell 2 so as to prevent unnecessary pumping. Further,as for light components included in probe light, the larger the rate ofa linearly polarized component within the whole components is, thebetter it is to the component included in probe light. Note that, aslong as probe light includes a linearly polarized light component, itmay include other polarized light components. Probe light generated bythe light source 11A and converted by the converter 12A is irradiated tothe gas cell 2 while being guided by, for example, an optical fiber orthe like. Probe light may be directly irradiated to the gas cell 2without using an optical fiber or the like by disposing the probe lightirradiation portion 1A inside the magnetic shield 7, for example.

The pump light irradiation portion 8 includes a light source 81 and aconverter 82. The light source 81 is a laser generation device thatgenerates a laser of a frequency that corresponds to a hyperfinestructure level transition of the atom sealed in the gas cell 2. Inother word, the light source 81 generates a laser that synchronizes witha wavelength that can excite gaseous atoms sealed in the gas cell 2. Theconverter 82 includes a circular polarization filter and the like, andconverts the laser having been generated by the light source 81 to pumplight having a circularly polarized light component. Pump light that hasbeen generated by the light source 81 and converted by the converter 82is irradiated to the gas cell 2 while being guided by, for example, anoptical fiber or the like. Pump light is irradiated so that itintersects with probe light in the gas cell 2. Pump light may bedirectly irradiated to the gas cell 2 without using an optical fiber orthe like by disposing the pump light irradiation portion 8 inside thegas shield 7, for example.

The magnetic field measurement apparatus 9A is characterized in that theincident direction of pump light having been converted to by theconverter 82 is configured so as to coincide with the axis of themagnetic shield 7.

2-2. Operation

FIG. 7 is a diagram for explaining the principle of magnetic fieldmeasurement by what is known as a two-beam type measurement apparatus.In the two-beam type measurement apparatus, when pump light isirradiated onto gaseous atoms sealed in the gas cell, the gaseous atomsare optically pumped and the probability distribution of magnetic momentgenerated when the energy changes takes a form corresponding to a regionR₂ that stretches along the incident direction of the pump light. Thisprobability distribution is called “orientation”. A vector thatindicates the direction of magnetization generated by the formation oforientation (hereinafter, referred to as a magnetization vector M)points to the direction of orientation. In this case, the irradiationdirection of the pump light is the direction of the magnetization vectorM.

As shown in FIG. 7, for example, if pump light having a circularlypolarized light component in an arrow D₇ direction is irradiated towardan arrow D_(p) direction (+y direction), orientation that is distributedin the region R₂ stretching in the +y direction is formed in the gaseousatoms through which the pump light passes, and the magnetization vectorM in the +y direction is generated. Then, if probe light having alinearly polarized light component whose electric field vibrationdirection is arranged along the arrow D₀ direction is irradiated towardan arrow D_(r) direction (+x direction), the orientation performs Larmorrotation central to the arrow D_(M) direction (−z direction)perpendicular to both the arrow D_(p) direction (+y direction) and thearrow D_(r) direction (+x direction). This causes the magnetizationvector M to rotate central to the z-axis; at this time the following twophenomena take place.

-   Phenomenon A: a phenomenon in which the orientation (magnetization    vector M) is forced to return to the direction of pump light.-   Phenomenon B: a phenomenon in which the orientation experiences    relaxation so as to have a spherical origin-symmetry form, which is    a form of orientation before optical pumping.

As a result, the magnetization vector M forms a trajectory in a spiralshape on an xy plane. FIGS. 8A and 8B are diagrams for explaining atrajectory of the magnetization vector M. For example, in theconfiguration shown in FIG. 7, the magnetization vector M forms atrajectory T_(M) as shown in FIG. 8A on the xy plane viewed while facingthe arrow D_(M) direction (−z direction). Further, the polarizationplane of probe light described above passes through the orientation androtates due to the Faraday effect. Accordingly, magnetic flux density inthe arrow D_(M) direction (−z direction) inside of the gas cell 2 isdetermined by measuring a rotational angle of the polarization planementioned above.

For example, in the case where the magnetization vector M forms thetrajectory T_(M) as shown in FIG. 8A, an expected value of magnetization<M_(X)> in the +x direction is obtained as a specified value. In thecase where the horizontal axis represents magnetic flux density B andthe vertical axis represents the expected value of magnetization <M_(X)>respectively in a graph, an output waveform as shown in FIG. 8B isobtained. The magnetic flux density B to be measured can be determinedby reading the output of a linearly changing portion in the waveform inthe vicinity of the origin (a range of ΔB in the drawing).

A relational expression between the expected value of magnetization<M_(X)> and the magnetic flux density B is as follows.

$\left. \frac{{\langle M_{x}\rangle}}{B} \right|_{B = 0} = {{{- C}\; \frac{\gamma}{\Gamma_{g}}} \approx {{- C}\; \gamma \; T_{2}}}$

Here, Γ_(g) is a relaxation rate and is given by the inverse of arelaxation time T. The relaxation time T is expressed in a formula of1/T=1/T₁+1/T₂, where T₁ is a longitudinal relaxation time and T₂ is atraverse relaxation time. Further, in the case where ω_(L) is a Larmorangular frequency [rad/s], γ is a gyromagnetic ratio [rad/sT], and B ismagnetic flux density [T], a formula of ω_(L=)γB holds. Since themagnetization vector M is a total of magnetic moment per volume unit, aproportional constant C is considered to be a parameter proportional toatomic density n.

Here, in order to explain the aforementioned features of the magneticfield measurement apparatus 9A according to the invention, operations ofa measurement apparatus without having the features mentioned above aredescribed first. FIGS. 9A and 9B are diagrams for explaining magneticfield measurement by the measurement apparatus without having thefeatures that are included in the magnetic field measurement apparatus9A. This measurement apparatus includes the same constituents as themagnetic field measurement apparatus 9A, but the incident directionD_(p) of pump light is configured so as not to coincide with the axisdirection of the magnetic shield 7. For example, in this measurementapparatus, the axis of the magnetic shield is arranged along the y-axisdirection shown in FIGS. 9A and 9B, and pump light having a circularlypolarized light component is irradiated toward the +z direction. Thatis, the incident direction D_(p) (+z direction) of pump light forirradiation is configured so as not to coincide with the axis directionof the magnetic shield (y-axis direction). Therefore, orientation isgenerated in a region R_(c) indicated in a broken line in FIG. 9A in thegaseous atoms sealed in the gas cell. This orientation has a form ofapproximately rotational symmetry central to the z-axis, and stretchesalong the z-axis direction.

Since this measurement apparatus includes the magnetic shield whose axisis arranged along the y-axis direction, a magnetic field disturbancealong the x-axis direction, the z-axis direction and a resultantdirection of the x-axis and y-axis directions can be suppressed at ahigh probability. However, a magnetic field disturbance along the y-axisdirection is likely to exert an influence on the hollow area of themagnetic shield. The orientation generated in the gaseous atoms sealedin the gas cell as described above rotates central to an axis parallelto the y-axis upon receiving the magnetic field disturbance along they-axis direction. That is, the orientation rotates so as to have aposture corresponding to a region R_(d) indicated in a solid line inFIG. 9A. FIG. 9B shows the orientation that is viewed while facing the+y direction. As described thus far, the orientation that has originallybeen generated in the region R_(c) indicated in a broken line is made torotate in an arrow D₈ direction influenced by the magnetic field in they-axis direction that has not been suppressed sufficiently by themagnetic shield, and consequently has a posture corresponding to theregion R_(d).

In the example shown in FIGS. 9A and 9B, in order to measure themagnetic flux density in the arrow D_(M) direction (−x direction), probelight is irradiated toward the arrow D_(r) direction (+y direction). Inother words, probe light is irradiated from the −y direction of the gascell, and an rotational angle of the polarization plane of the probelight having passed through the gas cell to the +y direction thereof ismeasured by a measurement unit (polarization separation unit 3, lightreceiving portion 4, signal processing circuit 5). As illustrated inFIG. 9B, since the orientation, when viewed along the +y direction,rotates from the region R_(c) to the region R_(d) upon receiving themagnetic field disturbance along the y-axis direction, the magnetizationvector M also rotates in the −x direction. Accordingly, themagnetization vector M forms a different trajectory from that in FIG. 8Adescribed before, and the expected value of magnetization becomes adifferent one, whereby a waveform becomes an output waveform influencedby the magnetic field disturbance. Therefore, the measurement in thiscase is likely to be influenced by a disturbance in magnetic field.

Meanwhile, FIGS. 10A through 10C are diagrams for explaining magneticfield measurement by the magnetic field measurement apparatus 9Aaccording to the invention. The axis of the magnetic shield 7 in themagnetic field measurement apparatus 9A is arranged along the y-axisdirection shown in FIG. 10A through 10C, and pump light including acircularly polarized light component is irradiated toward the +ydirection. That is to say, the incident direction D_(p) (+y direction)of the irradiated pump light coincides with the axis direction of themagnetic shield (y-axis direction). Accordingly, orientation isgenerated in the region R₂ as shown in FIG. 10A in the gaseous atoms inthe gas cell. This orientation takes a form of approximately rotationalsymmetry central to the y-axis and stretches in the y-axis direction. Inthis case, probe light is irradiated toward the arrow D_(r) direction(+x direction) so as to measure the magnetic field in the arrow D_(M)direction (−z direction).

Since the magnetic field measurement apparatus 9A has the magneticshield 7 whose axis is arranged along the y-axis direction, a magneticfield disturbance along the y-axis direction is likely to influence thehollow area. The aforementioned orientation generated in the gaseousatoms in the gas cell 2 rotates about the y-axis direction as an axisupon receiving the magnetic field disturbance along the y-axisdirection. In other words, the orientation rotates in an arrow D₉direction as shown in FIG. 10A. FIG. 10B shows the orientation that isviewed while facing the +y direction. As described above, theorientation generated in the region R₂ takes a form of approximatelyrotational symmetry central to the y-axis and stretches in the y-axisdirection. Therefore, even if the orientation is rotated in the arrow D₉direction following a trajectory of an arc central to the y-axis, itsform hardly varies. Accordingly, as shown in FIG. 10C, on the xy planeviewed while facing the arrow D_(M) direction (−z direction), thedirection of the magnetization vector M generated by the formation oforientation hardly differs between before and after the orientationrotates due to the magnetic field disturbance. Therefore, in this case,when probe light having a linearly polarized light component isirradiated toward the arrow D_(r) direction (+x direction), themagnetization vector M forms a trajectory as illustrated in FIG. 8A,then the magnetic flux density is determined. That is to say,sensitivity of the magnetic field measurement apparatus 9A is unlikelyto be influenced by the magnetic field disturbance.

The entire disclosure of Japanese Patent application No. 2011-229036,filed Oct. 18, 2011 is expressly incorporated by reference herein.

What is claimed is:
 1. A magnetic field measurement apparatuscomprising: a magnetic shield that is formed in a elongated hollow shapehaving openings at both sides thereof; a gas cell in which gaseous atomsare sealed and that is disposed in a hollow area of the magnetic shield;an irradiation unit that irradiates irradiation light including linearlypolarized light adjusted so that a vibration direction of an electricfield coincides with an axis direction of the magnetic shield onto thegaseous atoms sealed in the gas cell along a direction perpendicular tothe axis of the magnetic shield; and a measurement unit that measures arotational angle of a polarization plane of the irradiation light whichhas been irradiated by the irradiation unit and passed through thegaseous atoms.
 2. The magnetic field measurement apparatus according toclaim 1, wherein the irradiation unit guides the irradiation light tothe gas cell with an optical fiber.
 3. A magnetic field measurementapparatus comprising: a magnetic shield that is formed in a elongatedhollow shape having openings at both sides thereof; a gas cell in whichgaseous atoms are sealed and that is disposed in a hollow area of themagnetic shield; a pump light irradiation unit that irradiates pumplight including a circularly polarized light component onto the gaseousatoms sealed in the gas cell along a direction parallel to an axis ofthe magnetic shield; a probe light irradiation unit that irradiatesprobe light including linearly polarized light onto the gaseous atomssealed in the gas cell along a direction perpendicular to the axis ofthe magnetic shield; and a measurement unit that measures a rotationalangle of a polarization plane of the probe light which has beenirradiated by the probe light irradiation unit and passed through thegaseous atoms.
 4. The magnetic field measurement apparatus according toclaim 3, wherein the pump light irradiation unit guides the pump lightto the gas cell with an optical fiber and the probe light irradiationunit guides the probe light to the gas cell with an optical fiber.